This function is currently implemented by providing a hold button on the telephone set. The procedure is to depress the hold key and then replace the handset of the first telephone set on-hook. The function of the circuit is to maintain the first telephone set connected to the telephone line until the handset of the second telephone set has been taken off-hook, and then to disconnect the first telephone set.
This function is currently carried out by a voltage detection circuit which is implemented as a discrete circuit in the telephone set. This existing parallel phone detection circuit detects the voltage variation which occurs on the telephone line when the second telephone set has its handset taken off-hook. It then disconnects the first telephone set from the telephone line in response to the detection of this voltage variation. However, this circuit is not reliable when used in noisy environments in which voltage spikes can occur on the line. It is also costly because it requires a discrete circuit in the telephone set.
Moreover, the detection of voltages is unsatisfactory for the reason that telephone systems in different countries have different voltages. It is therefore necessary to tailor the detection threshold to individual country requirements.
The present invention seeks to provide a parallel phone detection circuit which does not suffer from these disadvantages.